This is a noise making device which uses compressed air to make a loud deep noise. My favorite use for it is to scare trick-or-treaters when they knock on my door. The sound that it produces depends on the overall dimensions and shape of the structure, but it typically starts with a high pitch squeal followed by a very deep bass noise. I used a "Y" splitter so I could have 2 sources of noise. You could probably make plenty of noise with just a single head. I'm not much of an academic musician, so I don't know if what genre of instrument this ends up being (horn? percussion?), but it uses vibration against the end of a tube to create the sound. You can press on the edge of the membrane to "bend" the noise a little.

My inspiration for this project came from a balloon organ I saw at Maker's Faire in Austin, TX. What I saw was a fairly small organ made from PVC with compressed air and a balloon vibrating membrane.  What I remember of that stuck with me and I eventually found a video of Fran Holland's copper piping balloon organ. I just scaled that up to a much larger level. I initially used this as a stationary noise maker but this year, I adapted it to a more portable format powered by CO2 paintball tanks.

I should mention a little bit about SAFETY. Compressed air can be dangerous. Wear eye protection. If using CO2, it is even more risky as the tanks used contain much higher pressure. You could get frostbite from a rapidly discharging tank should something go wrong. You should use as little air pressure as you can to get whatever desired effect you're looking for. The membrane heads (and hose clamps) WILL FLY OFF if you send way too much air pressure to them all at once and they will hurt if they hit you. Use an air pressure regulator on your air compressor or be very careful using a CO2 tank (with multiple valves). For CO2 tanks, the best I could do at regulating air pressure was to just barely crack the pin valve so that the least amount of gas flow was enabled and mid-hose was a heavy duty ball valve. I don't (yet) go into much detail about your air supply, but just be careful.

Step 1: How It Works

This section covers the general theory of how it creates the tone. The design involves 2 different PVC pieces, one fitting inside the other one. The reducer piece creates a separate air gap section around the inner ring. Air enters the small section from the side and as it goes against the membrane into the large air column, the membrane vibrates. This is where the sound comes from.

Compressed air could come from an air compressor (used for air tools) or a portable CO2 tank (as used in paintball). Each has its strengths and weaknesses. The compressed air is preferable if your setup is stationary. Most air compressors come with a regulator built in that you can easily lower down to around 30-50psi. A 10gal air tank will make about 1 minute of noise before the compressor kicks in to start refilling the tank. Conversely, a 20oz CO2 tank will last roughly 3-4 minutes.

As for the design of the air column, I've used a 6ft straight pipe for the loudest, deepest effect.  The portable version includes a 180deg turn with a 90deg turn at the end for aiming purposes. This is nowhere near as loud, but makes sense for portability.

There are several grades of PVC. The heavy grade stuff will cost you a lot more money but will be much louder. Lighter PVC (used for sewage/drainage) is better for portable applications as it is MUCH lighter. There are techniques on the internet that describe how to bend PVC using heat application. I don't go into those here, but they are easy enough to find online. Tapping the holes for the air hose adapters may be more of a challenge with the thinner PVC.

Step 2: Gathering the Parts

The list of items needed to build this device:

Air column tubing:
   4" pvc tubing about 6' long for stationary. Build to suit for portable version.

Resonator section PVC:
  4" Y-splitter

PVC reducer
  3" down to 2" reducer. Make sure it fits inside of the 4" tubing. I had to Dremel mine a little bit if I recall correctly.

Large hose clamp to fit around 4" tubing

Air hose adaptors

Air hose

Air hose "T" splitter

Air valve for compressed air version:
  Use automated water sprinkler valve for <90psi air. This valve requires several adapters to get it down to 1/4" air tubing.

Air valve for CO2 version:
  use a heavy duty ball valve. No normal/cheap air regulator will work with CO2 due to the high pressure.

Membrane materials:
  This one may take some trial and error. I went with a hybrid material found at my local hardware store. You want it thin but tough. It should have some elasticity but not so much that it fails after only a little bit of use.

Step 3: The Resonator Head

Take some spare inner tube rubber (or electric tape, whatever is convenient) and wrap the 3"-2" reducer enough that it fits snug into the main opening. You want it to be air tight.

Drill and tap a hole in the side of the main "Y" PVC piece. Make sure that it's positioned above where the 3"-2" reducer narrows (see image below).

Position the membrane material over the whole top and place a hose clamp over it to lock it down. Pull on the edges of the membrane after lightly tightening the hose clamp to make it taught. Once it's relatively smooth around the edges, tighten the hose clamp enough to secure it but not enough to damage the material.

The resonator head should be ready at this point. If you have a "Y" section, repeat for the second opening.

Step 4: Air Supply

The stationary setup can use a standard garage air compressor. A 12V automated sprinkler valve can be used to toggle air supply to device. This is how I've scared children on Halloween for the past 3 years.

The portable setup can use CO2 (such as from a paintball gun). You must use a ball valve with CO2 instead of a check valve. Most regular air tools will fail under the much higher psi of a CO2 tank. The ball valve is the only mechanism that I didn't destroy in the process of setting up the CO2 tank.

Step 5: Stationary Version

This version is simple. Merely place the resonator head atop a long section of PVC. If you can, it may be a good idea to "tune" the tube to maximize resonance. You'd need a slightly larger tube to slide up and down the far end of the main tube. I use the 12V electric sprinkler valve to switch air to it in this configuration. Hide it on your front porch and hit the switch when someone rings the doorbell. :)

Step 6: Portable Version

The overall assembly of the portable version was much more challenging. I used the existing resonator head from a previous project on this one. That one piece was notably heavier than the rest of the thinner tubing that I used for the main chassis. I bought an adapter to scale up from the thin 4" to the thick 4" material.

I also zip tied a small camelbak backpack to it and taped a loop for a belt to the bottom. This was relatively comfortable. I had to use a separate adaptor system for the CO2 tank that was borrowed from a friend's paintball equipment.
Even better, un-wire your doorbell from the button and wire the solenoid valve into the bell-push so then when they ring the doorbell, it sounds the thing :)
I would love to hear a recording of this!
I'll take some video of it before putting it away for the season. Unfortunately, the bass noise is much lower than most cameras will effectively record.

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